Occupant detection systems using a heating element for both seat heating and occupant detection are known. These systems operate by periodically disconnecting a heating current source from the heating element so the occupant detection circuit can use the heating element as an occupant sensor. A more detailed description of occupant detection systems can be found in U.S. patent application Ser. No. 12/433,923 filed by Kincaid et al on May 1, 2009, the entire disclosure of which is hereby incorporated herein by reference. The occupant detection circuit outputs an excitation signal to generate an electric field about the heating element. The presence of an object or occupant near the heating element influences the electric field and so influences an electric impedance of the heating element as measured by the occupant detection circuit. The heating control circuit used to disconnect the heating current source from the heating element must isolate the heating current source from the heating element to prevent interference with the occupant detection circuit. If the heating control circuit influences the electric field impedance, the accuracy and reliability of occupant detection is reduced.
It has been proposed to use transistors for connecting and disconnecting heating current to and from the heating element. When the transistors are off the heating current is disconnected. However, the transistors have open-switch impedance that remains coupled to the heating element. Thus, the heating element is not isolated from the heating current source and the accuracy of the occupant detection circuit measurement is affected. It has also been proposed that the effect of the transistor open-switch impedance on the occupant detection circuit can be reduced by adding diodes in series with the transistors and biasing the diodes to reduce diode junction capacitance. However, the diodes increase circuit complexity and do not isolate the open-switch impedance because the circuit does not eliminate diode junction capacitance. Thus, adding diodes does not isolate the heating circuit source from the occupant detection circuit. It has also been proposed to use mechanical relays for connecting and disconnecting heating current to and from the heating element. Unfortunately, the relay contacts and their interconnecting wires still remain connected to the heating element even when the relays are opened, thereby adding open-switch impedance to the measurement made by the occupant detection circuit, and reducing the accuracy of the measurement.